Space weather means the state of the space environment and is usually
expressed in terms of the behaviour of energetic particles, as well
as in changes in electric and magnetic fields. We are mostly interested
in conditions in near-Earth space, though space weather is important
throughout the solar system.

The significance of space weather lies in its potential impact
on man-made technologies on Earth and in space, for example, on
satellites and spacecraft, electricity power grids, pipelines, radio
and telephone communications and on geophysical exploration. Space
weather also has implications for manned space flight, both in Earth
orbit and further out into space.

Space weather arises as a result of various dynamic, but relatively
short-lived, phenomena produced by the Sun, which are carried in
the solar wind and which may interact with the Earth's magnetosphere (see our on-line glossary for other common terms). Long-term changes, that is, changes over
decades to centuries, also occur in the near-Earth space environment.
These changes are due to long period variations in solar magnetic
activity and in variations in the strength
of the Earth's magnetic field. However, for the most part we
are concerned only with those space weather 'events' that happen
on time scales of minutes to a few days. The implications of long-term
variations in the Sun's magnetic field, for example on possible
connections with climate change, is still being actively discussed
by scientists.

Dynamic solar-terrestrial events and interactions can be seen
in ground-based geomagnetic observations, for example those made
by BGS at our UK observatories.
Fast, high amplitude variations in geomagnetic measurements caused
by space weather are therefore sometimes described as posing a 'geomagnetic
hazard'. Space weather effects and the associated geomagnetic hazard
can be recognised in observatory records dating back over 150 years.

One of the more spectacular space weather events was the 13th
March 1989 solar and geomagnetic 'super storm'. This event caused
a widespread electricity blackout for over nine hours, and affected
6 million people, in the region of Canada covered by the Hydro-Quebec
power grid. As a result of this event, power companies worldwide
are now much more aware of the risk posed by geomagnetic storms.

In all these activities, accurate and reliable real time data can
be very important and therefore some BGS data products, such as
our estimate of the Ap geomagnetic
index, are updated every few minutes.